A planetary gear comprises a planet wheel carrier, a sun shaft, a gear ring, and planet wheels supported by the planet wheel carrier so that the planet wheels are meshing with the sun shaft and with the gear ring. In cases where the gear ring is stationary, the gear ratio between the sun shaft and the planet wheel carrier is DR/DS+1, where DR is the diameter of the gear ring and DS is the diameter of the sun shaft. The planet wheel carrier in its simple form comprises a first end-section for supporting first ends of the shafts of the planet wheels and a second end-section for supporting the second ends of the shafts of the planet wheels. In many cases, however, a planet wheel carrier of the kind described above can be mechanically too elastic, i.e. not sufficiently stiff, and thus the positions of the planet wheels with respect to the sun shaft and to the gear ring are not necessarily as desired especially during high load conditions. This may cause excessive wear of the teeth of the planet wheels, the teeth of the sun shaft, and the teeth of the gear ring.
In order to increase the mechanical stiffness, many planet wheel carriers comprise a support structure that is between the first and second end-sections in the axial direction of the planet wheels. An inconvenience related to the support structure is that it requires room and thus the support structure limits the maximum diameter of the planet wheels especially in cases where there are four or more planer wheels. Limiting the diameter of the planet wheels sets an upper limit to the gear ratio because the diameter of the sun shaft DS cannot be smaller than:DR−2×DPmax,where DPmax is the greatest possible diameter of the planet wheels. Therefore, there is a trade-off between the mechanical stiffness of the planet wheel carrier and the maximum achievable gear ratio.
Publication JP2007071273 discloses a planet wheel carrier comprising a support structure that does not essentially limit the maximum achievable gear ratio but only if the number of planet wheels is at most three. However, in many cases, there is a need for at least four planet wheels.